The present invention relates to an offset network that operates to offset an input modulation signal to regulate output modulation. More particularly, the invention relates to DC offset network for offsetting a high frequency modulating signal being fed into a double balanced modulator, in order to enhance amplitude modulation of the carrier.
One of the more traditional methods of modulating a carrier is by means of a double balanced modulator, also referred to as a ring modulator. In a conventional double balanced modulator, four diodes are connected in a ring, such that the anode of one diode is connected to the cathode of the following diode. Thus, current flow is possible in one direction only around the ring. Two transformers are provided with their secondary windings connected to the electrically conjugate pairs of bridge terminals, and their primary windings available for input or output connections. Center taps on the secondary windings may be used for the application of a higher voltage, alternating current, input signal, which alternately commutes the conductive state of a pair of non-adjoining diodes in the ring. If a lower voltage input signal is applied to one of the transformers--which then becomes the input transformer--the commutating action of the higher voltage signal will cause polarity changes of the signal output from the other transformer--which then becomes the output transformer.
If the higher voltage input signal is a high frequency carrier signal and the lower voltage input signal is a modulating signal of a lower frequency, then, under normal operating conditions, and with perfect symmetry of all elements, the output signal will contain only the sum and the difference components of those two frequencies. That is, a double sideband, suppressed carrier, amplitude modulated signal.
In view of the characteristic carrier frequency suppression effected by double balanced modulators, such circuits provide poor amplitude modulation of the carrier signal. The suppression of the carrier frequency with respect to the sideband frequencies, (i.e., the carrier frequency plus or minus the modulating frequency), reduces the carrier modulation. The present invention is directed to an apparatus and method for enhancing the amplitude modulation of the carrier signal that is obtainable from the output of a dual balanced modulator. The invention is intended to have particular application where the carrier frequency signal is in the high frequency range, i.e., greater than one GHz and the modulating frequency is also in the high frequency range, i.e., greater than 10 MHz.
It is known that biasing the modulating signal will regulate the part of the carrier cycle during which the diodes operate in a low impedance condition. The bias signal may also be useed to compensate for the irregularities of the conductivity of the switching diodes such that differences in the conduction characteristics of each diode do not result in degradation of the output signal.
Though other systems have proposed providing a bias signal to regulate the operation of diodes in a dual balanced modulator, none of those systems have disclosed or suggested the use of such a bias signal to enhance the percent modulation of the carrier frequency signal. Additionally, prior double balanced modulators incorporating bias signals are typically designed to facilitate low frequency operation and are unsuitable for use in the high frequency ranges in that the present invention finds application, e.g., to simulate a multiplexed radar return signal, that is amplitude modulated in accordance with the angle of arrival of the radar return signal.